<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .item {
      position: relative;
      width: 200px;
      height: 60px;
      border: 1px solid red;
      counter-increment: item-counter;
    }

    .item::after {
      position: absolute;
      left: 50%;
      width: 30px;
      height: 20px;
      color: #000;
      content: counter(item-counter);
    }
  </style>
</head>

<body>
  <ul>
    <li class="item"></li>
    <li class="item"></li>
    <li class="item"></li>
    <li class="item"></li>
    <li class="item"></li>
    <li class="item"></li>
  </ul>
</body>

</html>